Inertial electromagnetic throat microphone



Aug. 22, 1950 b; w. MARTIN 2,519,916

. INERTIAL ELECTROMAGNETIC THROAT MICROPHONE Filed April 30, 1945 M 2Sheets-Sheet L Y )7 I Fz 1 z- J 29 IN V EN TOR.

ATTORNEY? w/zmw. BY 6\ a Patented Aug. 22, 1950 INERTIAL ELECTROMAGNETICTHROAT MICROPHONE Daniel William Martin, Indianapolis, Ind., as-

signor to Radio Corporation of poration of Delaware America, a cor-Application April 30, 1945, Serial No. 591,070

8 Claims.

This invention relates to vibration translating devices and moreparticularly to a sound powered throat microphone of the inertia typeadapted to be supported externally in contact with the throat adjacentthe larynx of the user for translating voice sound waves or vibrationsinto corresponding signal currents.

An object of the invention is to provide a small low mass microphone ofthe inertia type capable of generating sufficient signal voltage inresponse to speech vibrations, transmitted through the neck of a user tothe casing of the microphone adapted to be worn in contact with thethroat, to operate satisfactorily one or more telephone receiverswithout the use of external power such as amplifiers and batteries incircuit, as well as to operate properly with amplifiers.

Another object of the invention is to substantially improve the emciencyand linearity of response, and thereby the intelligibility of devices ofthis type, without increasing size and weight. Certain devices in theprior art use push-pull magnetic structures which eliminate evenharmonic distortion components, but in the invention the odd harmonicsare also eliminated due to the use of a balanced armature.

Another object of the invention is to improve the construction of suchdevices in the way of ruggedness, simplification, adaptability toquantity production, serviceability, and low cost considering theperformance obtained.

Still another object of the invention is to provide a sound poweredthroat microphone of the inertia type having improved performance as aresult of a, novel magnetic unit in which magnetic and dynamic symmetryin a double air gap arrangement are combined with a rectilinearly movingmagnetically balanced armature. According to the invention, thiscombination is free from electrical response to spurious non-rectilinearvibratory movements. For example when rocking occurs, the displacementsof the ends of the armature in the two gaps are equal in magnitude andopposite in direction. Because the magnetic fields in the two gaps arealso opposite in direction and equal in magnitude, there is no fluxvariation in the armature due to the rocking motion, and therefore nocontribution to the electrical output. In conventional single gaparrangements and unsymmetrical double gap Eirrangements, rocking motionintroduces double frequency distortion components into the electricaloutput.

Further, in accordance with the invention the vibratory device isadapted to operate in contact with selected portions of the body of theuser, such as the neck through which sound vibrations are transmittedfrom the throat. It comprises a casing containing a magnetic armatureand an associated field magnet system including a permanent magnet, polepieces and signal translating coil. The armature is secured to thecasing by stifiness means for cophasal vibratory movement therewith andthe magnet system is suspended from the casing by compliance means, therelations of the mass of the casing and armature to that of the magnetsystem, together with the characteristics of the stiffness andcompliance means, being such that there is relative vibratory movementbetween the armature and pole pieces in accordance with sound vibrationstransmitted through contact to the casing. The mass of the magnetsystem, the inertia member, exceeds that of the casing and armature, the

driven member. V

In a preferred modification, using a magnetically balanced armature, incombination with magnetic and dynamic symmetry, substantially only truerectilinear vibratory movement of the armature, i. e. in a directionnormal to its plane or length, generates signal voltage and current,spurious response due to rocking vibratory movements being balanced out.

In the drawings: Fig. 1 is an enlarged end elevation view partly insection of the device embodying the invention, the increased scale sizebeing of the ratio of about one to three,

Fig. 2 is an enlarged side elevation view in section of the deviceembodying the invention, the

section being taken on the lines II-II of Fig. 1,

Fig. 3 is an enlarged plan view of the device with top cover removedshowing the interior, partly in section, as viewed on line III-III ofFig. 1, this view being drawn on a, smaller scale than are Figs; 1 and2, the dimensions being twice those of an actual working model fromwhich the drawings were made,

Fig. 3a is a top plan view of certain of the elements of Fig. 3 removedby sectioning, shown in elevation in Fig. 1, while Fig. 3b is a planview of a flexible elementpartly visible in Fig. 3,

Fig. 4 is an enlarged view in perspective of the device embodying theinvention, the top cover being removed, and the scale being slightlysmaller than in the case of Fig. 3,

Fig. 5 is an enlarged plan view of a modification embodying theinvention, with portions of the casing removed to show the interior,

Fig. 6 is a further enlarged View, in perspective, of the magnet systemshown in Fig. 5,

Fig. 7 shows characteristic curves of performance of the devices shownin different figures,

Fig. 8 is a block diagram showing a system in which a device embodyingthe invention has been successfully employed,

Fig. 9 is a diagrammatic plan view, partly in section, showing a form ofthe invention that is preferred, from the standpoint of performance,with parts removed to show the interior,

Fig. 10 is a side elevation View partly in section taken on lines X-X ofFig. 9 prior to removal of parts,

Fig. 11 is an end elevation view of the device, partly in section, takenon lines XIXI of F g. 10.

Referring now to the drawings, Figs. 1 to 4, an inertia type contactmicrophone comprises a molded casing f insulating material including abase portion l and a cover 3. In the casing is mounted a bar typearmature 5 of highly permeable magnetic material, and'a field magnetsystem consisting of a block magnet 1 of material having a high magneticenergy density, pole members 9, pole pieces ll, l3, l5, I1, anda signaltranslating coil [9 surrounding the armature 5. The casing is ofrelatively low mass, rigid in construction, and is substantially airtight for the purpose of excluding dirt and magnetic particles as. wellas reducing undesired air vibrations or external noise. However, theconstruction of theinner mechanism is such that entrance of airvibrations would have little or no effect. The casing 2 can easily bemade water-tight by sealing the joints with suitable material. Themagnet material of high magnetic density is an alloy of about 8%aluminum, 14% nickel, 24% cobalt, 3% copper and rest iron and isdescribed in Proc. IRE, October 1942, pages 437 to 440 by Ruder, also inG. E. Review, volume 45, Number 4, April 1942 by B. M. Smith. It carriesabout twice the magnetic flux of ordinary magnet material comprisingaluminum, nickel, cobalt and steel, heretofore widely used, and hasapproximately'three times as much available magnetic energy. It cannotbe made and magnetized in horseshoe shapes because a D. C. orundirectional field must be applied during heat treating period. It isavailable only in bars or blocks with a linear magnetic axis. Itdisplays its unusual magnetic properties only along the axis where thefield was applied. Applicanthas taken advantages oi its shape andunusual characteristics and has adapted it in novel arrangement ofmagnetic parts to a small efficient or highly sensitive transducer.

The armature 5 is rigidly secured within the casing by means of supportelements such as bolts 2! and nuts 23. These support elements have ahigh degree of stifiness so that the armature moves in cophasalvibratory relation with the casing. The bolts are locked in place in thecasing by a clamping action resulting from athreaded engagement with acompliant support element 25 of brass, the bolt threads engagingcorresponding internal threads in holes 21 (Fig. 3b) in the ends of theflexible arms, respectively, of the compliant element 25. These ends'ofthe arms acting as nuts for the bolts, are clamped tightly againstintegrally molded seats in the lower casing. Likewise, the compliantelement 25 is held in place in the casing and the magnet field system isbolted securely in place on the body portion of this compliant member bymeans of bolts 29 the ends of which engage threaded holes 31 in thecompliant member, a spacing element 32 being interposed between themagnet system and member 25 to provide clearance for the flexible armsof member '25.

The compliance of these arms of element 25 together with the mass of themagnet system is such relative to the lesser mass of the casing andarmature that there is relative vibratory movement between the armatureand the pole pieces, later described. The resonant period of the devicecan be made diiferent for various applications by using compliantsupport members of different resilient material or thickness or by otherchanges in design. For example, in one case the microphone is used inannounce systems employing loudspeakers. The compliant member was madeof different material and design to give a substantially higher resonantperiod, thereby attenuating lower audio frequencies, a desirable featurefor actuating speakers.

' The block magnet! is provided with a pair of mild steel highlypermeable interconnecting members 9 for completing the magnetic circuitthrough pole pieces ll, l3, I5 and I1, thereby eifecting a saving inspace and magnet material and obtaining better magnetic efl'ects thanmight be expected from a horseshoe shaped magnet of other material andcomparable size. Flat plane faces on element 9 engage corresponding flatpolar north and south faces, respectively, of magnet 1, therebyestablishing equipotential magnetic surfaces at the ends of the magnetsystem members. By having the flux turn the corner within the members 9instead of within the magnet member 1, greater efficiency and a savingof magnet material is effected. Members 9 are provided with bottom fiatfaces at right'angles to the first-mentioned faces disposed in a planespaced below and parallel to the bottom face of said magnet and toitsmagnetic axis for engaging the top face of a lamination element of polepiece ll. Each pole piece is made up of a group, say four, laminationsriveted together by rivets 33. Laminal pole piece groups H and I5 areseparable from the remaining structure as shown in Figs. 3 and 3a,whereas the laminae composing pole pieces I3 and I! are riveted to acommon plate 35, as of brass, for ease of assembly, along with a spacinglaminae 37. The different groups 01. laminae constituting the four polepieces are spaced by laminae 3'! a distance such that the verticalspacing, or air gaps, between their inwardly extending end portions isgreater than the thickness of the armature 5. Whereas in the prior artthe air gaps were formed by bending the inwardly extending pole pieceportions, in this arrangement he pole pieces, and component laminations,are all flat and of like shape for production and are easily assembled,with the resultant air gap formed as above mentioned. The lower edges ofthe spool ends 20 of the translating coil extend through an opening inthe base plate 35 and bears against spacer 32 and the body of flexibleelement 25. The upper edges of the spool support the magnet in spacedrelation from the top pole pieces I i and !5. While the magnet isprevented from siliding downward by means of the coil form it is limitedin any upward tendency to movement by means of a non-magnetic flexibleclamp 39 of Phosphor bronze held in place by the heads of the screws 29in the complete assembly.

True rectilinear balanced magnetic armature action is obtained by reasonof the fact that the inwardly extending portions or the pole pieces 5,overlap in such manner that pole pieces of like polarity are atdiagonally opposite ends of the armature, and the steady flux traversesthe two gaps in relatively opposite directions. Referring to Fig. 1, thepole piece l and its inward extending portion is, say a south pole,while the pole piece l3 and its inwardly extending portion is a northpole. At the other end of the armature, not shown, the inwardlyextending portion of pole piece ll beneath the armature is of southpolarity while the inwardly extending portion of pole piece ll,overlapping the top of the armature, is of north polarity. Thisarrangement is more clearly shown in Fig. 5 and it is partiallyillustrated in Figs. 2 to 4. I

It will be understood that when the armature is in a neutral position inthe two gaps, centered midway between the overlapping pole piece ends,there is negligible flow of magnetic flux in the armature which is,therefore, free of steady flux saturation. As the armature movesrelatively to the pole pieces in an upward direction from the midposition there is a flow of magnetic flux longitudinally through thearmature from north pole piece H to south pole piece I 5 in onedirection through the armature by reason of the air gap to those polepieces being shortened. As the armature moves downwardly from midposition to shorten the air gaps to pole pieces [3 and H the flux flowsin the opposite direction longitudinally through the armature. As theflux flows through the armature, signal current flows in one directionin the translating coil as a result of induced potential, the currentflowing in the opposite direction as the flux flow in the oppositedirection through the armature. This action will be readily understoodby those skilled in the art and need not be further elaborated upon,attention being called to a broadly similar magnetic circuit arrangementina loudspeaker disclosed in Brigham Patent 1,533,372, April 14, 1925.

One novel advantage of this arrangement as applied to inertia typemicrophones is that only rectilinear vibratory movement induces usefulsignal currents whereas undesired rocking movement, which wouldordinarily produce spurious response, is substantially balanced out andinduces no current. This is illustrated in Fig. 7 where curve Arepresents the response characteristic in decibles versus audiofrequency in cycles per second, of the device shown in Figs. 1 to 4. Thestiifness of the compliant support and the mass of the magnet systemdetermines the resonant points. Curve B illustrates the characteristicproduced by rocking movements in other arrangements, for example thatshown in Figs. 5 and 6 hereinafter described. Curve A illustrates theperformance characteristic to the sound powered device of Figs. 1 to 4.Curve B shows the performance of a similar device but with a stiffercompliant support for causing the first resonant peak to occur at ahigher frequency, thereby attenuating the lower frequencies. Curve Crepresents the same device as does curve A but in combination with thecapacitor and resistor circuit elements of Fig. 8.

Figs. 5 and 6 show an earlier form of the present invention. Fig. 5shows a permanent magnet block I of Alnico V with the pole pieces H',l3, l5 and I1 attached directly to the polar ends of the block magnet 1without the use of intermediate separate connecting members as inFig. 1. Spacing elements 31' are disposed between pairs of pole piecesof like polarity for spacing same in diflerent planes, the ends ofadjacent unlike ones constituting active air gaps in which the armature5' shown Fig. 5 is disposed. The pole pieces extend laterally andinwardly relative to the magnet and easing, instead of beneath themagnet as in Figs. 1 to 4. The magnet system, including the block ofmagnetic material, the pole pieces and the translating coil [9, aremounted upon an assembly plate 35' and mounted upon a flexible spidermember 25'. The center portion of the spider member is apertured andanchored to the casing around a central raised seat or boss 4 molded inthe casing for rigidly supporting one end of the armature 5'. The outerends of the flexible arms of the spider member 25' are attached to theplate 35' supporting the magnet system. The other end of the armature isattached to an inwardly extending seat 6 on the wall of the lowerportion I of the casing. While this arrangement results in a shallowerconstruction and has certain of the advantages of the preferredmodification, it lacks the dynamic symmetry of the arrangement of Figs.1 to 4. The stiffness of the compliant element 25' was so chosen as toproduce a main resonance above 800 cycles, as in the case of curve B,but the lack of symmetry produced a characteristic B (Fig. 7).

In one actual use, the preferred form, of Figs. 1 to 4, is used as asound powered throat microphone in combination with a sound powered typeof telephone headset. Reference is made to patent Anderson et a1.2,345,966, April 4, 1944, over which this invention is an improvement.The equipment carried by one user will consist of a throat microphoneand a headset adapted to be connected in circuit with similar equipmentworn by other users, resulting in ready communication, particularly inwartime on land or on the water, without the necessity for amplifiersand batteries. In other arrangements, however, amplifiers for obtainingstill better results are employed, arrangements being made for readyswitching to sound powered operation in case of failure of amplifiersand/or ower supply sources. Equipment known as MI-2466 (later supersededby MI-2478) consisted of a sound powered throat microphone and headsetand included a volume control 24 with the headset together with plug-inconnections for an amplifier. See Fig. 8. Sound powered communication isprovided for emergency (power off) conditions as shown by the positionHi for the switch in Fig. 8. In order to realize higher signal to noiseratio in extremely high noise levels an amplifier 22 is used between themicrophone 2 and earphones 26 for better operation. The amplifier 22with a 500 ohm shunt resistor in its input has a capacitor 8, of 0.1mid. in series with its input to reduce the low frequency transmissionfrom the throat microphone. Th capacitor is not included in the circuitused for sound powered operation for the reason that the lower levelsinvolved the additional low frequency response obtainable without thecapacitor, is found to be advantageous. In some cases, the capacitorplus the higher resonance point, previously described, were used withamplification for best results as to intelligibility in ,systemsemploying loudspeakers.

Still better results from dynamic symmetry can be obtained if the magnetsystem is compliantly supported in a plane with the air gaps. Such anarrangement is illustrated in Figs. 9 to 11 which may also serve toillustrate some of the features inherent inone devise of Eig 1. It wouldhave: been prefer-able in Fig. 1 to have: supported the magnet. systemin the mannerof Fig. 2 but overall size limit requirements did notpermit of this arrangement and somewhat of a compromise. had to be made.The advantage of this. susnens en is that it achieves a cancellation ofthe efiec e f. rocking vibratory motion from the output. signal to aneven higher degree than in the species of Fig. 11. In this arrangementone of the SEX-icin laminae i9 between the pole pieces is provided withan integral flexible extension which engages in looking relation theside wall o the oa i The armature and this special separation member arelocked in the sam plane, an advanta e us relation in that the. axis ofrotation is actu lly in the plane of the gap while it, was Q -llyparallel to it and passing through the center line, of the unit. in thearrangement of Fig, 1.

Qutput energy due ,to rocking motion or vibration parallel to the planeof the ap was. leak aneed out to a first. approximation in the structureof species in Fi 1 because of the. fact hat there, existed a substantialam un of s mm (gaps equally spaced from center, armature sup= ports.equallyspaced from eenteri sp in suppo ts equally spaced from eenten,equal gaps, center-e armature, mass of structure balanced about an axisin the. armature), and as a esult f h v n the magnetic field opposit ndi ection. n the wo aps the. sprin su orts, however moved o. th plane ofarmatur the u 1 voltage caused by rock g vo itn i balanced i to a stillreater de ree th in, t e ase f Fi l- The ar an e ent sho n n Elsi 9 hase n simpl fied by Q sion of tain details time trated in the, o herfigures The truc ures oi Fi s. 1 and 9 ha e other use th n threa micophone The are a ap d o h o noe d o various vi ratory dev es. nder teslso be used as. a t lenh ne re ei er; for xaeh ple, f; he bone c nductin tyne also have, utility as. eleetrio l n ek ns for mus eal m t ornents.v

While the inv ntion has been desc d, or t e purp ses f illustrat on it eec to ar ew lar stru ures, it i l he u der hat o e modifi ati s a d u esl me appa ent o those in :in t e a a d it in ended o be limi ed o y byth s p of the ol in d l -n5:

claim as my invention:

1. In a microphone of the inertia type responsive to voice frequencysound waves for transla-. tion thereof into corresponding electriccurrents, a housing adapted for operative direct contaot with a selectedspeech transmitting portion of the human body, an armature carried bysaid housing and secured thereto for cophasa-l vibrae tory movementtherewith, a high inertia member comprising a translating coilsurrounding said armature, a magnet and pole pieces disposed inoperative balanced magnetic relation to saidare. mature, compliant meansattached to said housing at points substantially in a plane with saidarmature and supporting said coil, magnet and pole pieces for relativevibratory movement with respect to said armature said pole pieces com?prising two pairs of oppositely polarized pole pieces forming air earswith s id armat re and being disposed at each extremity of said armaturesuch manner that rectilinear vibration of said arma u e r la ive to sai1301 p o s induce a substantial voltage in said coil, t e arr n ementand desi n f the microphone parts being she-r aoterized by su stantialdynam c s mm try ole magnetic relation to said armatur & tiveto said wereas re ls-ins movement balane s and induces sub tan ially n v lta inSaid 0011;. l a miereeliene oi the in rtia r ess nsive to vo oe requewrs und Waves for ire-1 51 tion it o i to o esoon ns ele t ca rrents a:hou n hav n rela i e ylow mas a d arran d o, he h d in d reet qe ltawith e 5 l o ed sr-eechtra ni n n n o th human be in an armatu tame bysa d us n an so ured hereto for eeonasal ibra or m ement h withiao inert.m om r sin a r slaure- QQil su roun ng said. rmature a ma net and polen eces, dis osed n operati alanc d eome n one s a sa nt Subat aoiied tosai s .ti l plane W Y. e. A V portin said qoil role i c s for at vevibratory niov out th res ec to sa d oreole p eces of like relaritv a ds o ed o oppoite side o sa d armat e a va en o p i xt e f-eel, th S ofsaid housing and t e emnl ant means there re ul relativev hratorymovement e een solid ammo hire an sa d ole mos angin e voice G er ia tpe for g ne a so i c rr nt a orator Qa i a field agi tor ern u e wi hisaid ca e oemrrisins one; magne of high reten-ti mategial ole piecesforming a or an gags are a t ansla ng @9 ou o aid. sys em a be tre armaur d ss i- @611- within sa d sail and ai san is alanced magneticrelation to and mounted in pendently of; system, mean t r n ati sa d eiure transve sely o sa d sio and, i dl there teat snared. oo etsf s es-ei r o m mea erewi h realize m ans tta h d to said easing a spared otherPoint in su n i l he same l ne as s d fi st-na e Points d s id s es forem-shea h pqr es' i net syst m whe e to eanc the efi s of rock-, ins vbratory moti, a ontaet mierorhene of th inertia typ an enc osed. housinfo ex ludin air r o s, a lgar type armature paroled within said housingnd scour dthe r opllas v b t ry movement therewith, translating coildisposed in s-note relat on to sai a matu e a Sheri c tangular blool;magnet havingfirst and second sol roses planes norma to it ma neti a aoppos e end f the block a e r ,rn er o masrietieel v pe m a le mater alonga-gig ea f aid face a p um ty o plane l minati s of mag etica yermeable ma e ial en i s i members and stacked upon each of said membersin spaoed parallel relation to a neutral face of a d magnet cert. o sa dl minati s of pposed polarity having portions extending inward 1 iron;said role meme & Overlapp relation ard sash other a 1 r la ve to one endof s d to? l n active r gap whioh' said one nd o s id armature iscenterse, thers or said i o pos polarit ha ing port on ext nd-1e nward ytoward e ch ot er .i. erlap n a e at t e other ed o s id mmense sesa eac ve 9 air gap in which said other end of said armature is centered.

5. In a contact microphone of the inertia type, an enclosed housing forexcluding air vibrations, a bar type armature carried within saidhousing and secured thereto for cophasal vibratory movement therewith, atranslating coil disposed in magnetic relation to said armature, a shortblock magnet of high retentivity material having a linear axis withfirst and second pole faces on opposite ends of the block, a metallicpole member of magnetically permeable material engaging each of saidfaces, a plurality of plane laminations of magnetically permeablematerial engaging said members and stacked upon each of said members inspaced parallel relation to a neutral face of said magnet, certain ofsaid laminations of opposed polarity having portions extending inwardlyfrom said pole members parallel to the magnetic axis of said magnet andin overlapping relation toward each other relative to one end of saidarmature for forming an active air gap in which said one end of saidarmature is centered, others of said laminations of opposed polarityhaving portions extending inwardly toward 'each other in overlappingrelation at the other end of said armature for forming a second activeair gap in which said other end of said armature is centered, theportions of like polarity being disposed at diagonally opposite ends ofsaid armature whereby only rectilinear vibratory movements of saidarmature induce signal currents in said coil.

6. In a transducer, an armature, a translating coil, a short blockmagnet having a linear magnetic axis with first and second pole faces atopposite ends of the block, a metallic pole member of magneticallypermeable material for engaging each of said faces, a plurality of planelaminations of magnetically permeable material enga ing said members andstacked upon each of said members in spaced parallel relation to abottom face of said magnet parallel to its magnetic axis, certain ofsaid laminations of opposed polarity having portions extending inwardlyfrom said pole members in overlapping relation toward each other andrelative to one end of said armature for forming an active air gap inwhich said one end of said armature is centered, others of saidlaminations of opposed polarity having portions extending inwardlytoward each other in overlapping relation at the other end of saidarmature for forming a second active air gap in which said other end ofsaid armature is centered, the portions of like polarity being disposedat diagonally opposite ends of said armature whereby only rectilinearvibratory movements of said armature induce signal currents in saidcoil, said armature extending in transverse relation to said magneticaxis.

7. In a transducer, an armature, a translating coil, a short blockmagnet having a linear magnetic axis with opposed pole faces at oppositeends of the block, a metallic pole member of magnetically permeablematerial for engaging each of said faces, said members having fiat facesengaging said pole faces and having second faces in a plane at rightangles to said fiat faces said second faces being spaced below a neutralface of said block magnet, a plurality of flat pole pieces ofmagnetically permeable material engaging said second faces and stackedupon each other in parallel relation to said neutral face of saidmagnet, certain of said pole pieces of opposed polarity having portionsextending inwardly from said pole members in, overlapping relationtoward each other for forming an active air gap in which one end of saidarmature is supported centrally, others of said pole pieces of opposedpolarity having portions extending inwardly toward each other inoverlapping relation at the other end of said armature for forming asecond active air gap in which said other end of said armature issupported centrally thereto, and resilient means for flexibly supportingsaid magnet block and said pole pieces with said air gaps centeredaround opposite ends of said armature for relative vibration thereto.

8. In a microphone of the inertia type, a housing, an armature carriedby said housing and secured thereto for cophasal vibratory movementtherewith, a translating coil surrounding said armature, a magnet andpole pieces disposed in operative balanced magnetic relation to saidarmature, compliant means attached to said housing and supporting saidcoil, magnet and pole pieces for relative vibratory movement withrespect to said armature, said pole pieces comprising a pair of opposedpole pieces forming an air gap disposed at each extremity of saidarmature in such manner that rectilinear vibration of said armaturerelative to said pole pieces induces a substantial voltage in said coilwhereas rocking movement balances out and induces substantially novoltage in said coil, said compliant means being substantially in thesame plane with said air gaps.

DANIEL WILLIAM MARTIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,731,905 Memnich Oct. 15, 19291,830,755 Fitch Nov. 10, 1931 2,111,643 Salvatori Mar. 22, 19382,255,249 Greibach Sept. 9, 1941 2,328,222 McCarty Aug. 31, 19432,371,973 Minton Mar. 20, 1945 2,441,975 Robertson May 25, 1948

